Until recently, the main service of the cellular network is voice service, for which traffics in the forward and reverse links (also known as downlink and uplink, or DL and UL for short) are almost balanced. Meanwhile, high rate data-service gets a quick boost nowadays and has become a major share of the cellular network service market. Such trend leads to very fluctuating load imbalance between UL and DL in a broadband cellular network.
In view of the status quo of a fixed UL/DL radio resource splitting such as a fixed bandwidth and time slot ratio in current FDD/TDD systems, the new generation of cellular network is thus facing a challenge of catering the time-varying demand on radio resource due to the dynamic traffic on both forward link and reverse link between a base station and a user equipment.
In wireless systems such as W-CDMA HSPA evolution and LTE-A, quite a few of important features are introduced to improve the efficiency of the spectrum utilization: e.g., multiple antenna technologies, inter-cell interference coordination, advanced receiver, relay technology, etc. However, the advanced technologies mentioned above do not solve such load imbalance problem directly.
Currently, it is more and more identified that a full/partial UL/DL radio resource pooling or flexible resource splitting is indispensable in future advanced air-interface because of more and more prominent traffic volume emerging in current and future radio services.
At present, for LTE TDD system, it is being widely discussed to dynamically change the TDD configurations in accordance with the DL/UL traffic imbalance. According to the proposed solution, more sub-frames are allocated for the link whose traffic is heavier. That is, the UL/DL radio resource splitting is substantively adjusted according to the traffic volumes on UL and DL.
FIG. 1 schematically illustrates the UL/DL radio resource pooling before and after the above mentioned radio resource adaptation scheme. As illustrated in FIG. 1(a), before radio resource adaptation, DL is heavily loaded with traffic while UL happens to be half-loaded. That is, the DL radio resource is exhausted while a significant part of the UL radio resource is unused. After adjusting the radio resource ratio between UL and DL by changing the TDD configuration according to the traffic volumes on UL and DL, the UL/DL radio resource splitting is substantively adjusted, as illustrated in FIG. 1(b).
Although there are no realistic measurements from the practice reported so far, the following shortcomings of such a solution can be predicted:                This solution is only applicable to the LTE-TDD based system.        There is additional DL/UL interference between adjacent eNodeBs with different TDD configurations due to such TDD configuration adaptation. Specifically, due to a high transmission power of radio access points (RAP), a DL transmitting RAP can result in performance degradation a victim RAP suffers in UL reception. Therefore, the benefit due to the adaptation of the UL/DL radio resource is offset by this RAPs interference problem. Though there might be some mitigating method to partially deal with this drawback, this problem still is a major obstacle to substantial and constant improvement of the system performance.        There is UE to UE interference when UEs served by cells with different TDD configurations are close together.        Possible impact on user experience during the TDD-configuration change.        There can be a high cost to solve the system stability problem for large scale deployment for LTE-TDD system or, in order to reduce the development and deployment cost, such technology is only deployed in some limited scenarios.        